1. Field of the Invention
This invention relates to a hydrophilic polymer and a method for the production thereof. More particularly, it relates to a hydrophilic polymer such as a water-soluble or water swellable polymer having a small residual monomer content and a method for the production of the polymer from a hydrated gel polymer.
2. Description of the Prior Art
Such water-soluble polymers as polysodium acrylate, polyacrylamide, partial hydrolyzates of polyacrylamide, and polyvinyl alcohol have the ability to produce a hydrated gel polymer on being subjected to solution polymerization. These water-soluble polymers have been finding utility in water-treatment grade flocculants, oil-excavation grade sludge additives, food additives, and tackifiers, for example.
Examples of the water-swellable polymers which, on being subjected to solution polymerization, produce a hydrated gel polymer include cross-linked polyacrylates, saponified acrylic ester-vinyl acetate copolymers, modified cross-linked polyvinyl alcohols, partially neutralized cross-linked polyacrylates, cross-linked isobutylene-maleic anhydride copolymer, and starch-acrylic acid graft polymer They have been finding utility in sanitary absorbents such as sanitary articles and disposable diaper, water-retaining agents for agriculture, horticulture, and forestation, and films for preserving foodstuffs. They have a prospect of further growth in demand in the future. In these applications, the polymers have the possibility of touching the human skin and even being absorbed in the human body. When they are discarded after use, they develop the possibility of straying in the environment and eventually mingling into drinking water. The demand for minimization of the residual monomer content in the polymer is gradually gaining in impetus in recent years.
For the purpose of decreasing residual monomer contents in hydrated gel polymers, there have been proposed methods which attain the decrease by addition of ammonia and amines (Japanese Patent Publication SHO 33(1958)-2,646 and Japanese Patent Laid-Open SHO 50(1975)-40,689) and addition of sulfites and hydrogensulfites (U.S. Pat. Nos. 2,960,486 and 4,306,955) to residual monomers, methods which accomplish the decrease by polymerizing residual monomers by combined use of low-temperature decomposition type and high-temperature decomposition type initiators (Japanese Patent Publication SHO 50(1975)-44,280 Japanese Patent Laid-Open SHO 59(1984)-133,205, and Japanese Patent Laid-Open SHO 53(1978)-141,388) and combined use of redox initiators and azo type initiators (Japanese Patent Laid-Open SHO 50(1975)-96,689 and U.S. Pat. No. 3,573,263), and a method which attains the decrease by decomposition of residual monomers with microorganisms (Japanese Patent Publication SHO 60(1985)-29,523).
The addition of ammonia, amines, sulfites, and hydrogen-sulfites is fairly effective in decreasing residual monomers. Insufficient use of these compounds is futile. Further, use of these compounds poses a problem that these added compounds are left manifesting their inherent toxicity The methods resorting to combined use of initiators are not sufficiently effective in decreasing residual monomers The method relying on microorganisms has a disadvantage that it is not sufficiently feasible economically.
A conduction heat-transfer type drier, a radiation heat-transfer type drier, or a hot-air heat-transfer type drier can be used for producing a water-soluble or water-swellable polymer by drying a hydrated gel polymer. By reason of fastness of the work of drying, the hot-air heat-transfer type drier (hereinafter referred to as "hot-air drier") finds popular acceptance. The method thus resorting to the heat-transfer drier, however, cannot remove the residual monomers sufficiently, although water contained in the polymer can be removed effectively.
A method has been proposed for the production of a water-soluble polymer by the polymerization of an aqueous solution of 40 to 80% by weight of acrylamide or a mixture of acrylamide with other vinyl monomer, which method comprises depositing the aqueous solution in the form of a film on a solid support and polymerizing the film of the aqueous solution under a polymerization atmosphere having a relative humidity kept at a level exceeding 60% (Japanese Patent Laid-Open SHO 54(1979)-155,296). The method of this nature, however, can be applied only to the polymer polymerized by the specific polymerization method and further it is very difficult to attain the highly decreased residual monomer content.
An object of this invention, therefore, is to provide a hydrophilic polymer and a method for the production thereof.
Another object of this invention is to provide a hydrophilic polymer such as a water-soluble or water-swellable polymer having a low residual monomer content and a method for production of the polymer mentioned above from a hydrated gel polymer.